We have modeled a new, low cost polyimide Capacitive Micromachined Ultrasonic Transducer (CMUT) device constructed using a polyimide membrane. This paper presents the preliminary results of the electromechanical modeling of this new device. Polyimide was chosen as the construction material because it is chemically and thermally stable, it has excellent dielectric properties; it is durable and low cost; and, it is compatible with standard integrated circuit processing techniques. The fabrication technique utilizes the simple, low cost PolyMEMS fabrication process developed at Virginia Commonwealth University. We present the electromechanical model for an ultrasonic MEMS detector based on a polyimide CMUT. The sensor consists of a membrane, suspended at two ends, and two electrodes. Deflection of the membrane is measured by the change in capacitance between the two electrodes. We have modeled the device to determine the amount of deflection given a known pressure differential. This information is used to determine the minimum, maximum and nominal capacitance that the device will generate. We are also in the process of modeling the modes of vibration of the device. We have fabricated test structures in the form of bridges to aid in the optimization of the fabrication process.